Session app triggers an AttributeError

[loic@…]

I suspect this occurs when session has expired, or something similar and may be related to a change of API in django 1.4:

AttributeError at /
'QuerySet' object has no attribute '_prefetch_related_lookups'

Environment:

Request Method: GET

Django Version: 1.4
Python Version: 2.7.3
Installed Applications:
('django.contrib.auth',

'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.sites',
'django.contrib.messages',
'django.contrib.staticfiles',
'xxx'
'django.contrib.admin')

Installed Middleware:
('django.middleware.common.CommonMiddleware',

'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware')

Traceback:
File "xxx/Django/django/core/handlers/base.py" in get_response

188. response = middleware_method(request, response)

File "xxx/Django/django/contrib/sessions/middleware.py" in process_response

36. request.session.save()

File "xxx/Django/django/contrib/sessions/backends/db.py" in save

52. session_data=self.encode(self._get_session(no_load=must_create)),

File "xxx/Django/django/contrib/sessions/backends/base.py" in encode

79. pickled = pickle.dumps(session_dict, pickle.HIGHEST_PROTOCOL)

File "xxx/Django/django/db/models/query.py" in getstate

65. len(self)

File "xxx/Django/django/db/models/query.py" in len

88. if self._prefetch_related_lookups and not self._prefetch_done:

Exception Type: AttributeError at /
Exception Value: 'QuerySet' object has no attribute '_prefetch_related_lookups'

DB is MySQL.